What EPA's radon map says about Johnson County

Johnson County is mapped as Zone 2 - Moderate Potential by EPA — meaning the county-wide average indoor level is predicted between 2 and 4 pCi/L. Johnson County sits at the 42nd percentile nationally for radon potential — above 42 of every 100 rated counties.

How Johnson County compares

About 13,320 people live in Johnson County, Illinois, spread over 343.737 square miles. That works out to roughly 39 people per square mile. Among counties sharing Johnson County's EPA zone, it outranks 27% of them by population. Johnson County falls in EPA Radon Region 5, one of 524 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in Johnson County

Call it a middle case: above the 1.3 pCi/L most US homes average, below the 4 pCi/L line EPA uses to recommend mitigation — that's Johnson County's Zone 2 prediction. A Zone 2 rating is a moderate county-wide average, not a guarantee about any one house in Johnson County — testing is the only way to know an individual home's actual level.

If Johnson County tests above the action level

Mitigation cost doesn't depend on Johnson County's zone — it depends on the house's foundation. A typical system runs $800–$2,500 (about $1,200 typical), and testing first is what tells you whether that's needed at all.

Find Illinois's state radon program

Johnson County's test kit answers today; a Airthings radon monitor keeps answering for Johnson County over time. Affiliate link, RadonZoneCheck earns a commission there, at no extra cost to you. Use code RADONZONE-10OFF at checkout for 10% off.

More on how this number was built

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. The guide to what a radon zone actually means covers why testing matters in every zone, not just Zone 1.